Get the big bag. The panniers don't come out unless you unzip the side pockets and pull them out. Plenty of room for a pair of shoes, jacket, sandwich, etc in the main compartment, which expands on the top. I had one, but gave it to a guy at the coffee shop whom only travels by bike.
Get the Topeak rack. The Topeak bags fit on it, and you can always attach other types of bags to it. My old trunk bag from Performance velcro straps right on it just like a rack that's not Topeak.